1. Technical Field of the Invention
The present invention relates generally to electric fuel pumps that include a pump portion and a motor portion. More particularly, the invention relates to an electric fuel pump that has an improved structure by which the pump can supply fuel at a high flow rate.
2. Description of the Related Art
There is known, for example from Japanese Patent First Publication No. 2005-110477, an electric fuel pump for an internal combustion engine which includes a pump portion and a motor portion. The pump portion sucks fuel from a fuel tank into pump chambers, pressurizes the sucked fuel in the pump chambers, and discharges the pressurized fuel out of the pump chambers via a discharge outlet to the engine side. The motor portion is configured as an inner rotor-type brushless motor and functions as a drive source to drive the pump portion.
More specifically, as shown in FIG. 8, the electric fuel pump includes a tubular metal housing 101 that receives both the pump portion 102 and the motor portion 103. In the housing 101, the pump portion 102 is located at a front end of the housing 101, and the motor portion 103 is located in a central portion of the housing 101. Further, at a rear end of the housing 101, there is provided a synthetic resin-made end cover 104. In addition, all of the pump portion 102, the motor portion 103, and the end cover 104 are fixed to the housing 101 by crimping both the front and rear ends of the housing 101.
The pump portion 102 includes an impeller 105, which is configured to be driven by the motor portion 103, and a pair of front and rear pump casings 106 and 107 that together receive and rotatably support the impeller 105. The motor portion 103 includes a cylindrical rotor, which is configured to drive the impeller 105 of the pump portion 102, and a hollow cylindrical stator that is disposed to surround the radially outer periphery of the rotor.
The rotor includes a rotating shaft 108, to which the impeller 105 of the pump portion 102 is fixed, a rotor core 109 mounted on the rotating shaft 108, and a permanent magnet 110 that is arranged on the rotor core 109 to form a plurality of magnetic pole portions. In addition, opposite ends of the rotating shaft 108 are respectively rotatable supported by the rear pump casing 107 and end cover 104 via bearings 111 and 112.
The stator includes a stator core 113, stator coils 114, and insulating bobbins 115. The stator core 113 is disposed to surround the radially outer periphery of the magnet 110. The stator coils 114 are wound around teeth of the stator core 113 via the insulating bobbins 115.
The housing 101 encloses the radially outer periphery of the stator core 113, thereby preventing fuel from leaking in the radially outward direction. A suction inlet 121 is formed in the front pump casing 106. The pump chambers 122 are respectively formed between the front pump casing 106 and the impeller 105 and between the impeller 105 and the rear pump casing 107. A pump-motor communication passage 107 is formed in the rear pump casing 107. An annular gap is formed in the motor portion 103 between the outer periphery of the magnet 110 and the inner periphery of the stator core 113; the gap makes up a fuel passage 124 through which the fuel discharged out of the pump portion 102 passes the motor portion 103 toward the discharge outlet 126 formed in the end cover 104.
In operation, the impeller 105 is rotated along with the rotating shaft 108 upon energizing the stator coils 114 of the motor portion 103. With rotation of the impeller 105, fuel is sucked from the fuel tank into the pressure chambers 122 via the suction inlet 121, pressurized in the pressure chambers 122, and discharged out of the pressure chambers 122 via the pump-motor communication passage 123. Then, the fuel flows through the fuel passage 124 formed in the motor portion 103 and a discharge passage 125 formed in the end cover 104. Thereafter, the fuel is discharged out of the electric fuel pump via the discharge outlet 126 to the engine side.
However, with the above-described electric fuel pump, it is difficult to meet the requirement of supplying fuel to the engine at a high flow rate when the engine is used in a recent four-wheel motor vehicle. This is because the cross-sectional area of the fuel passage 124 formed in the motor portion 103 of the electric fuel pump is very small, and thus the pressure drop of the fuel across the motor portion 103 is very large.
To reduce the pressure drop across the motor portion 103, one may consider increasing the cross-sectional area of the fuel passage 124. However, in this case, the annular gap between the outer periphery of the magnet 110 and the inner periphery of the stator core 113 is accordingly increased, thereby increasing the magnetic reluctance between the rotor and stator of the motor portion 103 and decreasing the magnetic attraction between the same. As a result, the efficiency of the motor portion 103 is lowered, making it impossible to supply the fuel to the engine at a high flow rate.
To solve the above problem, Japanese Patent First Publication No. 2007-127013, an English equivalent of which is U.S. Patent Application Publication No. 2007/0098574, discloses an improved electric fuel pump. This electric fuel pump has, in addition to the annular gap formed between the outer periphery of the magnet and the inner periphery of the stator core, clearances provided between circumferentially adjacent pairs of teeth of the stator core on the radially outside of the annular gap. The annular gap makes up a first fuel passage, while the clearances together make up a second fuel passage. Consequently, the fuel discharged out of the pump portion can pass the motor portion through the second fuel passage as well as through the first fuel passage. As a result, the pressure drop across the motor portion can be reduced without lowering the efficiency of the motor portion.
However, for the improved electric fuel pump, there is still a limitation in supplying fuel to the engine at a high flow rate. This is because the clearances, which make up the second fuel passage, are provided between the radially inner ends of the teeth of the stator core, and there is accordingly a limitation in setting the clearances large.